The present invention relates to a method and to a device for producing a multicolor print on a sheet in printing presses. The sheet rests on the printing cylinder, for more than one revolution of the printing cylinder, is held by a gripper connection on the printing cylinder and is sequentially printed in several colors.
A printing press is known from U.S. Pat. No. 5,496,787 which has a sheet feeder, along with a sheet feeding stack, elements for conveying a sheet to be imprinted to a feed line, and feed mechanisms for conveying sheets from the feed line to a printing cylinder having three print saddles. The former works together with at least one cylinder group, consisting of an offset cylinder with two operative areas, and a printing forme cylinder, also with two operative areas, and a delivery device containing delivery chains.
Two revolutions of the printing cylinder are required for producing a multicolor print. In this case, a sheet is fed only to every second print saddle. Moreover, a printed sheet is removed only from every second print saddle.
Following the application of the last color, the printed sheet is passed on to a chain conveyor and is deposited on a delivery stack.
This printing press has a limitation that, because of its construction, there are no chances for providing the sheet with a larger color effect by the application of additional colors.
DE 195 23 378 A1 has become known recently, and which discloses a printing press operating in accordance with the same principle as the printing press in U.S. Pat. No. 5,496,787. In accordance with the subject of DE 195 23 378 A1, a laser illustrating arrangement is provided for illustrating the printing plates in the press.
Better color prints, by means of the application of more than four colors, cannot be achieved with this printing press either.
It is the object of the present invention to provide a printing press in such a way that more than four colors can be applied to the sheet in the course of one passage through the press.
In accordance with the invention, this object is attained by the utilization of a printing cylinder that receives and holds the sheets to be printed for more than one revolution. Several printing units are directly associated with the printing cylinder. At least one transfer drum is also associated with the printing cylinder. An additional printing unit is also associated with the printing cylinder or with the transfer drum.
The present invention makes it advantageously possible to increase the range of employment of a printing press with a triple-size printing cylinder and doublesize offset and printing forme cylinders in that a first and a second transfer drum are arranged downstream of the printing cylinder, and the first transfer drum is provided with a rubber blanket. In this way, the first transfer drum can be used as an image transfer device, and a third printing unit for applying further colors to the sheet can be installed in the printing press.
It is not important whether the first transfer drum receives the print image directly from a printing forme cylinder or indirectly from an offset cylinder. It is essential that the transfer drum is equipped in such a way that it can perform several functions such as sheet conveyance, image conveyance and image transfer. The attainment of the object of the present invention requires only a small outlay, because it is integrated in a printing press which can print in four colors on a sheet during one passage.
Moreover, the arrangement of an application device for use with the second transfer drum for varnishing or for the application of an additional color to the sheet, increases the range of employment of the printing press.